Will Saletan writes about politics, science, technology, and other stuff for Slate. He’s the author of Bearing Right.

To longtime readers of Human Nature, this question should be, if you'll pardon the term, familiar. A few years ago, we looked at the science and ethics of "
The Love That Dare Not Speak Its Surname
." Then we examined the prevalence of inbreeding in
nature
. Then we considered the awkward question of why, if incest is too genetically risky to permit, maternity in your 40s
isn't
.

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Now biologist Hamish Spencer and political scientist Diane Paul, writing in
PLOS Biology
, have reviewed the history of U.S. laws against cousin marriage, along with their scientific basis. And again, the evidence raises unsettling implications.

They start with the statistical case against restricting cousin marriages:

[T]he National Society of Genetic Counselors (NSGC) convened a group of experts to review existing studies on risks to offspring and issue recommendations for clinical practice. Their report concluded that the risks of a first-cousin union were generally much smaller than assumed—about 1.7%-2% above the background risk for congenital defects and 4.4% for pre-reproductive mortality—and did not warrant any special preconception testing. In the authors' view, neither the stigma that attaches to such unions in North America nor the laws that bar them were scientifically well-grounded.

But Paul and Spencer point out that the data aren't clear-cut. First, "statistics on the risks associated with cousin marriage are necessarily averages across many traits, and they are likely to be different for different populations." And second, it's

inappropriate to extrapolate findings from largely outbred populations with occasional first-cousin marriages to populations with high coefficients of inbreeding and vice-versa. Standard calculations, such as the commonly cited 3% additional risk, examine a pedigree in which the ancestors (usually grandparents) are assumed to be unrelated. In North America, marriages between consanguineal kin are strongly discouraged. But such an assumption is unwarranted in the case of UK Pakistanis, who have emigrated from a country where such marriage is traditional and for whom it is estimated that roughly 55%-59% of marriages continue to be between first cousins. Thus, the usual risk estimates are misleading: data from the English West Midlands suggest that British Pakistanis account for only ~4.1% of births, but about 33% of the autosomal recessive metabolic errors recorded at birth.

In other words, the American calculations understate the risk for an already inbred population such as British Pakistanis. And calculations based on British Pakistanis overstate the risk for most American cousin couples. You can't draw a uniform line against cousin marriages based on science. Arguably, for the same reasons, you can't draw a uniform line against sibling incest.

The same case can be made against a uniform age of sexual consent. The authors point out,

Beginning in the 1860s, many states passed anti-miscegenation laws, increased the statutory age of marriage, and adopted or expanded medical and mental-capacity restrictions in marriage law. Thus, laws prohibiting cousin marriage were but one aspect of a more general trend to broaden state authority in areas previously considered private.

As Human Nature has noted before, the age of actual maturity
varies considerably
depending on the person and the type of maturity (sexual, cognitive, emotional) involved. Granted, lawmakers have to draw lines somewhere. But let's not pretend such consistent lines are consistently apt.

Moreover, Paul and Spencer raise a far more troubling problem: The increase in genetic risk caused by cousin marriages among British Pakistanis may actually be overstated, for a curious reason.

[F]or a variety of reasons (including fear that a cousin marriage would result in their being blamed for any birth defects), UK Pakistanis are less likely to use prenatal testing and to terminate pregnancies. Thus the population attributable risk of genetic diseases at birth due to inbreeding may be skewed by prenatal elimination of affected fetuses in non-inbred populations.

In other words, many of the birth defects cited by British politicians as grounds for restricting cousin marriages may actually be the result not of cousin marriage, but of failure to screen and abort defective fetuses. So, in addition to maternity in your 40s, we now have a second logical target for genetic regulation: If inbreeding is too dangerous, what about "
inflicting
" maladies on your children by failing to screen the embryos? If you know you carry bad genes—and particularly if you're at higher risk of passing down a serious disease than most sibling couples would be—shouldn't we police your procreation just as carefully?